The present invention relates to a beam shaping optical system that affects a cross-sectional shape of a light beam. In particular, the present invention relates to a beam shaping optical system used in a light emitting device or an imaging optical system.
U.S. Pat. No. 4,948,223 (hereinafter "Patent") discloses a light shaping optical system as shown in FIG. 7. The optical system includes first and second wedge prisms 1 and 2 that are arranged in order from an incident side (i.e., a side from which a light ray enters the optical system, the left side in FIG. 7). The beam shaping optical system operates to, for example, convert an elliptical-shaped light beam (for example, as emitted from a semiconductor laser) into a circular-shaped light beam. The prisms 1 and 2 have principal sections that are parallel to each other, where a principal section of a wedge prism is defined as a plane perpendicular to both of the two refractive surfaces of the wedge prism through which a light beam passes. In other words, the principal section is defined as a plane perpendicular to a ridgeline of the refractive surfaces.
This type of beam shaping optical system is generally included in an optical system of an optical disk device, a magneto-optic disk device, a laser beam printer, or the like in order to shape a cross-sectional shape of a light beam emitted by a light source such as a semiconductor laser so that the light beam can be used effectively, or in order to form a symmetrical beam spot on an object surface.
Further, in order to increase the speed of such devices (laser beam printers and the like) a plurality of light beams may be simultaneously used to read or write information. In this case, a multi-beam semiconductor laser (not shown) can be used as a light source. A multi-beam semiconductor laser includes more than three light emitting points arranged in a single element and principal rays from the light emitting points forms predetermined angles with one another.
However, since the conventional beam shaping optical system disclosed in the patent is designed for converting the cross-sectional shape of a single light beam, there is a problem in that, if there is a minor variation in an incident angle, due to difference in exit angles of light beams emitted from light emitting points of a multi-beam semiconductor laser, there is a relatively large change in angular magnification by the beam shaping optical system. The following TABLE 1 is based on data disclosed in the patent and shows a relationship between the incident angle .phi. (deg.) to the first prism 1 and the exit angle .omega. (deg.) from the second prism 2 to illustrate the occurrence of an exit angle error .delta. for variations in the incident angle .phi.. The TABLE 1 also shows an imaginary exit angle .psi., an angular magnification .gamma., a percentage difference .gamma.', and the exit angle error .delta.. The imaginary exit angle .psi. is defined as an exit angle when the angular magnification is constant, i.e., the imaginary exit angle .psi. is determined by multiplying the incident angle .phi. by a paraxial angular magnification .gamma..sub.0 (equal to 0.669041 in this case). The angular magnification .gamma. is the actual angular magnification and the percentage difference .gamma.' is a percentage difference between the actual angular magnification .gamma. at each incident angle with respect to the paraxial angular magnification .gamma..sub.0, that is, .gamma.'=(.gamma..sub.0 -.gamma.)/.gamma..sub.0. The exit angle error .delta. is obtained by subtracting the imaginary exit angle .psi. from the exit angle .omega..
TABLE 1 ______________________________________ Imaginary Angular % Exit Incident Exit Exit Magnifi- Difference Angle Angle .phi. Angle .omega. Angle .psi. cation .gamma. .gamma.' Error .delta. ______________________________________ -2.0 -1.33272 -1.33808 0.66636 0.40% 0.00536 -1.6 -1.06710 -1.07047 0.66694 0.31% 0.00337 -1.2 -0.80098 -0.80285 0.66748 0.23% 0.00187 -0.8 -0.53440 -0.53523 0.66800 0.16% 0.00083 -0.4 -0.26739 -0.26762 0.66848 0.08% 0.00023 0.0 0.00000 0.00000 0.66904 0.00% 0.00000 0.4 0.26774 0.26762 0.66935 -0.05% 0.00012 0.8 0.53584 0.53523 0.66980 -0.11% 0.00061 1.2 0.80422 0.80285 0.67018 -0.17% 0.00137 1.6 1.07286 1.07047 0.67054 -0.22% 0.00239 2.0 1.34173 1.33808 0.67087 -0.27% 0.00365 ______________________________________
Thus, the amount of change of the angular magnification with respect to a change in the incident angle is relatively large in the conventional beam shaping optical system. Therefore, when the conventional beam shaping optical system is used for a multi-beam optical system with a multi-beam semiconductor laser, bean spots formed on an object surface are positioned at irregular intervals even if the emitting points of the light beams are positioned at regular intervals.
Further, the exit angle error in the conventional beam shaping optical system cannot be counterbalanced by altering a rotationally symmetric lens such as a collimator lens or an objective lens of the optical system. Because the exit angle error contains distortion that are represented by even order functions of the incident angle (as shown in FIG. 3), whereas distortion of a rotationally symmetric lens such as a collimator lens or an objective lens is represented by third or higher order, odd order functions.